Geophysicists Find Molten Zone Beneath Tibet

Pool of Magma Could Explain Why Tibetan Plateau Is Flat

An international team of geophysicists has discovered an enormous pool
of partially molten granite, a possible result of the Indian subcontinent's
thrust into Asia, in a band 100 miles wide, north to south, underneath
southwestern Tibet.

The existence of a partially molten middle crust beneath Tibet had been
proposed in the early 1970s on the basis of comparisons between the Himalayas
and other mountain ranges, but until now no study had verified the hypothesis.

Such a partially molten region could explain how the Tibetan plateau has
remained so flat despite being wedged between the Asian continent and the Indian
subcontinent, which has been plowing under Asia for at least 50 million years.
Such a pool of liquified rock would absorb some of India's impact and lift the
Tibetan plateau, which has an average elevation of about 16,000 feet.

The research, reported in a series of five articles in the Dec. 6 issue
of Science magazine, is the result of an ongoing collaboration among more than
two dozen geologists and geophysicists in China and at universities in the
United States, Germany and Canada. Principal investigators include Zhao Wenjin
of the Chinese Academy of Geological Sciences; Douglas Nelson, professor of
earth sciences at Syracuse University; Larry Brown, professor of geological
sciences at Cornell, and John T. Kuo, the Ewing and Worzel Professor Emeritus of
Geophysics at Columbia.

Scientists need to do more work to understand how the pool of granite
came
to exist and how deep it might be, Professor Kuo said. The Indian continent's
subduction, or underthrusting, could be creating friction at the northern end
of the Indian plate, but that would not fully explain how the rock stays molten.
The pool might be deep-earth magma trapped by India in the upper crust, or the
end of the Indian plate could even have broken off, releasing pressure on
surrounding rock and causing it to liquefy. "Upper mantle heat could also be
causing this," Professor Kuo said. There is no recent history of volcanic
activity in the region.

The investigation, known as Project INDEPTH (International Deep
Profiling of Tibet and the Himalaya), uses a technique called seismic profiling,
in which seismic waves generated by dynamite charges are reflected from deep
earth structures and are captured by instruments that detect vibrations in
solids. Researchers also used magnetotelluric surveys, which measure
conductivity of the crust at depths of at least 12 to 18 miles, and passive
source studies, in which small earthquakes provided the seismic waves used to
detect deep earth structures.

The scientific team produced a 240-mile-long transect, or cross-section,
of the Himalayas, with some gaps, showing deep earth features of the crust to a
depth of about 50 miles. They found a series of "bright spots"--loud
reflections--at depths of about nine miles in an area nearly 60 miles north of
the central Himalayas. The team interpreted those bright spots to mean the
reflecting material is liquid, and at those depths the material is almost
certainly molten or partially molten rock, Professor Kuo said.

In earlier work, Project INDEPTH scientists found a mid-crustal
reflection at 15 miles deep at the southern end of a 60-mile-long transect,
falling to about 27 miles at the northern end of the transect. That reflection
is called the Main Himalayan Thrust, or MHT, and scientists believe it is the
fault along which India is currently slipping under southern Tibet.

A key finding of a second phase, and one that requires further
investigation, is that reflections from the MHT drop abruptly from about 15
miles to 30 miles directly beneath a feature called the Kangmar Dome, a belt of
high mountains in the central Himalayas. Researchers believe the dome may have
been formed by material extruded upwards from the pool of molten rock by the
leading edge of the Indian plate. The dome could thus indicate the northernmost
edge of the Indian plate.

The group conducted the first phase of its seismic explorations along a
60-mile transect southwest of Lhasa in the summer of 1992, and extended it 120
miles further north in a second phase in 1994 and 1995. A third phase is
scheduled to begin in 1998. The work has been funded by the National Science
Foundation's Geodynamic Program and by both the National Natural Science
Foundation and the Ministry of Geology and Natural Resources in China. The
second phase was also funded by the Deutsche Forschungsgemeinschaft and the
GeoForschungsZentrum Potsdam (GFZ), both German government agencies that fund
scientific research.

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